1. Field of the Invention
The invention relates to a tape-transport arrangement for tensioning and transporting a magnetic tape which is unwound from a first reel, is subsequently transported past one or more magnetic heads for magnetically recording and/or reading signals, and finally wound onto a second reel and more particularly to a tape transport mechanism having first and second rotatable reel spindles for supporting and driving the first and the second reel respectively; a rotatable capstan, which is disposed upstream from the magnetic head(s) as defined by the direction of transport of the magnetic tape, for accurately determining the speed of the magnetic tape to be passed along the magnetic head(s); a capstan pressure roller for pressing the magnetic tape against the capstan; a transport spindle which is disposed downstream the magnetic head(s) for obtaining the desired tape tension in the part of the magnetic tape which is disposed between the capstan and the transport spindle and which passes the magnetic head(s); a transport-spindle pressure roller for pressing the magnetic tape against the transport spindle, and electrical drive means for rotating at least the second reel spindle, the capstan and the transport spindle.
The problems associated with the conventional method of tape transport in magnetic tape equipment will be discussed briefly, after which the state-of-the-art with respect to the tape transport arrangements mentioned above will be discussed.
In conventional tape transport arrangements which are frequently employed because of their simplicity a capstan is used which is disposed downstream the magnetic heads, according to the direction of transport of the magnetic tape. The magnetic tape is unwound from the first reel (or feed reel), passes the magnetic heads and then the capstan, and is finally wound onto the second reel (or take-up reel). The terms "feed reel" and "take-up reel" are always used with reference to the direction of transport of the magnetic tape. Thus, in the case of transport arrangements in which the direction of transport of the tape can be reversed, the two reels will change names upon reversal of the transport direction.
A problem associated with the conventional tape-transport arrangement is that the cumulative angle of tape contact transversed by the magnetic tape before it reaches the capstan is comparatively great, namely of the order of 80.degree.. An angle of tape contact is to be understood to mean an angle over which a magnetic tape makes contact with a cylindrical body, viewed from the axis of said body. Between the feed reel and the capstan the magnetic tape, in the conventional tape transport arrangement, makes contact with several cylindrical bodies, such as guide rollers and pins as well as the magnetic heads. The cumulative angle of tape contact is to be understood to mean the sum of the angles of tape contact owing to the contact consecutively made with a number of cylindrical bodies by the magnetic tape during its transport.
The cumulative angle of tape contact is not only indicative of the friction to which the tape is subjected owing to its contact with the said cylindrical bodies, but it also influences the degree in which tape tension variations near the feed reel, as a result of irregularities when the tape is unwound, affect the tension of the magnetic tape at the location of the capstan. Because of the great cumulative angle of tape contact, tape tension variations near the feed reel will manifest themselves as tape tension variations which are a factor 1.5 to 1.7 greater at the location of the capstan.
Variations of the tension of the magnetic tape produce variations in stretch of the magnetic tape. These manifest themselves as variations in the speed with which the magnetic track which is recorded (or to be recorded) on the tape is moved past the magnetic heads. This results in an undesired variation of the frequency of the reproduced or recorded signal, which undesired variation is superimposed on this signal and is referred to as "wow and flutter".
The force exerted on the tape by the feed reel and the tape-up reel nearly always depends on the diameter of the reels, in particular in cassette equipment. In a magnetic tape cassette this force may vary by a factor of 21/2. As a result of this, the work to be performed by the capstan on the magnetic tape varies from negative for a full feed reel to positive for an empty feed reel. Consequently, when the take-up reel is still empty, the capstan first exerts a braking action on the magnetic tape, the braking action gradually decreasing as the take-up reel becomes fuller and at a given instant changing into a tractive action. This means that during playing of a magnetic tape the force exerted on the magnetic tape by the capstan varies substantially. The greatest braking force must be exerted at the beginning of the magnetic tape and the greatest tractive force at the end. As the force with which the pressure roller cooperates with the capstan is subject to practical limits, irregular slippage may result between the capstan and the magnetic tape both at the beginning and at the end of the tape transport. These variations in the speed of transport of the tape along the capstand also manifest themselves as wow and flutter. The wow-and-flutter effects may accumulate in an overall effect which for good cassette recorders should not exceed approximately 0.2% of the desired constant speed.
A further drawback of conventional tape transport arrangements is the impossibility of tape transport in the reverse direction.
2. Description of the Prior Art
Tape transport arrangements of the type mentioned in the preamble mitigate some of these drawbacks. Such a tape transport arrangement is for example known from German Auslegeschrift 1,474,273 to which U.S. Pat. No. 3,409,239 corresponds. An important feature of this arrangement is the use of a "feeding" capstan, that is, a capstan which is situated upstream of the magnetic head or magnetic heads as determined by the direction of movement of the magnetic tape. This yields a substantial reduction of the cumulative angle of contact between the feed reel and the capstan, so that the slip at the capstan is reduced. If for the sake of simplicity it is assumed that the capstan is connected to a flywheel with infinite inertia, the magnetic tape after the capstan is, in principle entirely free of tension variations due to irregularities during unwinding of the feed reel, while variations in tape tension resulting from irrregularities during winding onto the take-up reel are now reduced by the prevailing cumulative angle of contact between the capstan and the take-up reel because the tape-tension variations in the downstream part of the tape are divided by a value greater than 1 instead of being multiplied thereby. The use of a transport spindle in the case of a "feeding" capstan is necessary in view of the possibility of looping of the magnetic tape near the magnetic heads, for example owing to a brief sticking or momentary slowing down or stopping of the take-up reel. By the use of a transport spindle the requirements imposed on the uniformity of winding of the take-up reel can be less stringent. Moreover, the transport spindle can be used for producing a tractile tension in the magnetic tape. Thus, the critical part of the magnetic tape, namely the part which is located near the magnetic heads, is virtually isolated with respect to variations in tape tension which might be introduced as a result of imperfections in winding as well as unwinding of the reels.
In the previously mentioned tape transport arrangement U.S. Pat. No. 3,409,239 the capstan and the transport spindle are both coupled to identical flywheels which are constituted by pulleys of equal dimensions. These pulleys are coupled by a common elastic belt to a drive spindle on which a drive pulley is mounted. The elastic belt is driven by the drive spindle in such a way that the belt (in the running direction of the magnetic tape) runs from the drive spindle to the capstan, then to the transport spindle and then back to the drive spindle. By the increasing stretch in the belt between the capstan and the transport spindle the flywheel of the transport spindle is driven slightly faster than the capstan flywheel, because the diameter of the belt is slightly reduced as a result of stretch. Owing to the slightly faster drive of the transport spindle the magnetic tape between the capstan and the transport spindle is tensioned. Because of the symmetrical arrangement this tape transport is suitable for both running directions.
The difference in speed between the capstan and the transport spindle is very small, of the order of 0.2%. As a result, if the capstan and the transport spindle each exhibit an oscillation of 0.002 mm and these oscillations are moreover in phase opposition, the tape tension in the part of the magnetic tape between the capstan and the transport spindle periodically varies from zero to a maximum, and in addition discontinues effects may occur. This may give rise to so-called "capstan wow" (approx. 7 Hz). This effect is likely to contribute some tenths of a thousandth to the overall wow and flutter in a cassette recorder.
Owing to the slight difference in speed between the transport spindle and the capstan and the occurrence of oscillations in the two spindles a sort of "starting effect" is produced when a tape recorder is switched on. The tape tension between the capstan and the transport spindle must, so to speak, be built up. This requires a few seconds. More sophisticated equipment with a tape transport arrangement of the type discussed herein may therefore incorporate an additional feature which ensures that recording or reproduction of the magnetic track on the tape, as the case may be, is started with a delay. However, this leads to great problems when two separate magnetic recordings should exzctly be joined exactly to each other.